1. Field of the Invention
The present invention relates to an optical filter for use in optical communications and other fields, and to a method of manufacturing the optical filter.
2. Description of the Related Art
Optical filters such as wavelength-selective pass filters (band pass filters) are widely used in optical communications. The band pass filters allow only light beams that are in a preset wavelength band out of all the light beams having reached the filter to pass.
One of the methods used in manufacturing optical filters of this type is to form a filter film on a substrate by vacuum evaporation, sputtering or the like.
FIG. 2 shows in schematic diagram an example of vacuum evaporation apparatus as a film forming apparatus. In FIG. 2, an evaporation apparatus 30 has therein a film forming area 1 for film formation by vacuum evaporation. In the area 1, a substrate holder 2 is provided to place thereon a substrate 3 on which a film is formed. Evaporation sources 5a and 5b are placed below the substrate holder 2. The components in the film forming area 1 is shown in section in FIG. 2.
Further, the evaporation apparatus 30 is provided with an operation unit (not shown). The apparatus is driven by operation of the operation unit, so that materials evaporated from the evaporation sources 5a and 5b are deposited on the substrate 3 to form a film (thin film) 4.
Arranged outside the evaporation apparatus 30 are a light source 7 that emits monitoring light, an optical fiber 8 for guiding the light emitted from the light source 7 to the evaporation apparatus 30, a light receiver 12, a computer 14, etc.
The monitoring light emitted from the light source 7 travels through the optical fiber 8 and enter the evaporation apparatus 30 from a window 9 provided in a lower part of the apparatus. The monitoring light entered from the window 9 travels straight across the film forming area 1 to reach the film 4 and the substrate 3 as indicated by A in FIG. 2. Some of the light passes through an optical fiber 11 provided above the evaporation apparatus 30 and reaches the light receiver 12.
A signal reflecting the amount of light that has reached the light receiver 12 is sent through a signal cable 13 to the computer 14, which controls the thickness of the film to be formed in accordance with the information of the signal.
To elaborate, the computer 14 measures, on the basis of the amount of light that has reached the light receiver 12, a change in one of two optical characteristics consisting of energy transmittance and energy reflectance when the monitoring light irradiates the film 4 that is in the process of formation. The measured change in energy transmittance, or energy reflectance, of the monitoring light is used to judge whether or not the film in process reaches the objective thickness. After it is judged that the objective thickness is attained, a signal for stopping the film formation is sent through a signal cable 15 to a driving unit (not shown) of the evaporation apparatus 30.
Receiving the signal, evaporation source shutters 6a and 6b cover right above the evaporation sources 5a and 5b, respectively, to stop the film formation.
A multi-layer optical filter in which thin films of different materials are layered can be manufactured by repeating the following operations. A material for forming an N-th layer (N is a positive integer) is evaporated from the evaporation source 5a to form the N-th layer. Subsequently, a material for forming an (N−1)-th layer, which is different from the material for forming the N-th layer, is evaporated from the evaporation source 5b to form the (N−1)-th layer. Thereafter, a material for forming an (N−2)-th layer, which is different from the material for forming the (N−1)-th layer, is evaporated from the evaporation source 5a to form the (N−2)-th layer.
FIG. 3 is a schematic diagram showing an example of the multi-layer film laminated as above. In FIG. 3, a multi-layer film having L layers in total is formed on the substrate 3 with a refractive index of ns. The layer the farthest from the substrate 3 is a first layer, the underneath layer is a second layer, layers below them are thus denoted in order and the nearest layer to the substrate is an L-th layer. The first layer has a refractive index of n1, the second layer has a refractive index of n2, a j-th layer has a refractive index of nj, the L-1 layer has a refractive index of nL-1, and the L-th layer has a refractive index of nL. The first layer has a physical thickness of d1, the second layer has a physical thickness of d2, the j-th layer has a physical thickness of dj, the L-1 layer has a physical thickness of dL-1 and the L-th layer has a physical thickness of dL. The optical thickness is obtained by multiplying the refractive index by the physical thickness. For example, the optical thickness in the j-th layer is njdj.
FIG. 4 is a graph showing the relation between the optical thickness and the energy transmittance in the case where a multi-layer film having three layers in total is formed on the substrate 3. It is understood from the graph that formation of each layer should be stopped at the extremal of the curve when n1d1=n2d2=n3d3=λ/4 is satisfied.
Prior art therefore judges that the energy transmittance reaches its extremal when the direction of change in energy transmittance (increase or decrease) is reversed. Alternatively, fitting or other processing is performed on the relation between the energy transmittance and the optical thickness (or time count during film formation which is almost in proportion to the optical thickness) obtained in the vicinity of the above extremal to put it into a simple equation such as quadratic curve or sine-wave curve in order to judge whether the extremal on the characteristic curve of, e.g., FIG. 4 is reached or not. When to stop the formation of the film 4 in each layer has been judged in those ways in prior art.
Methods similar to those methods of judging when to stop formation of each layer based on the change in energy transmittance have been employed in judging when to stop formation of each layer on the basis of the change in energy reflectance.
However, there are problems in the methods of judging when to stop formation of the film in which the extremal of the change in the amount of light is obtained directly and only from the change in energy transmittance or in energy reflectance as above. The problems include that it is only after the extremal is reached that the judging of the extremal can be made, and that making the judgement takes a while resulting in increase in the difference between the designed optical thickness and the optical thickness actually obtained.
To elaborate the above, it is common to make a computer or the like to judge that the extremal is reached only after the change in energy transmittance or in energy reflectance heads to the reverse direction for a certain period of time from the real extremal. This is intended to avoid erroneously judging a minute falter caused by noise or the like as the extremal. Thus the judgement of the extremal is made after a while from the real extremal. Also it takes no small amount of time to send, after the judgement is made, a signal for stopping the film formation to the film forming apparatus and to actually stop the film formation. Thus it has been impossible to prevent the difference between the designed optical thickness and the actual optical thickness from increasing.
When employing the method of using fitting or the like to judge that the energy transmittance or the energy reflectance reaches its extremal, on the other hand, it is important that the change in actual energy transmittance or energy reflectance should substantially coincide with the simple equation such as quadratic curve and sine-wave curve. However, the change and the equation substantially coincide with each other only when data of very narrow range in the vicinity of the extremal are used.
In addition, the actual energy transmittance or energy reflectance rarely makes a smooth curve and a change that seems like the extremal in a very narrow range often takes place. For that reason, a wrong point may be judged as the extremal, thereby increasing the difference from the designed thickness even more.